As the wireless communication industry continues to grow, the demand increases for smaller and smaller wireless communication devices. A wireless communication device also may be referred to herein as a “wireless device,” a “handset,” a “cellular phone”, a “mobile phone”, etc. Decreasing the size of a wireless device presents design challenges including the challenge of placing device components within a smaller volume. Additional challenges include maintaining acceptable levels of radio frequency (RF) energy radiated from the wireless device in a concerned near-field range while maintaining radiated efficiency of the device.
When the RF module is transmitting, a power amplifier magnifies an electronic signal and then broadcasts this modulated energy into the air through the antenna. A first portion of this energy travels through free space to the target destination. Another portion of this energy is dissipated as heat within the device. Finally, some of the energy is absorbed by objects near the device, including the user's body.
The power density surrounding an antenna of a wireless device varies as a function of, for example, distance from the antenna and orientation of the antenna. The fields around an antenna typically are divided into two regions. A first region is one near the antenna called the near field, and a second region is one at a large distance from the antenna called the far field. The boundary between the two is often taken to be at the radius, R, where R=2L2/λ, where L is the maximum dimension of the antenna and λ is the wavelength. The far field is the region where the radiation pattern is independent of distance from, e.g., a transmitting antenna. The near field is the region that is closest to the transmitting antenna and in which the field pattern depends on the distance from the antenna.
Limits for safe human exposure to radio frequency (RF) energy radiated from a wireless device in the near field are given in terms of a unit referred to as the Specific Absorption Rate (SAR). Specific Absorption Rate is an amount of radio frequency energy absorbed by the body when using a radio transmitter. The acceptable SAR level varies from country to country. The U.S. Federal Communications Commission (FCC) limits the amount of acceptable exposure to RF energy from a mobile phone to a specific local (or spatial peak) SAR level of 1.6 watts per kilogram of tissue (1.6 W/kg). When the wireless device is operating within these limits, the device poses no hazards to users. Thus, compliance with applicable near-field exposure limits must be considered in any mobile phone design.
The SAR levels of various phones vary due in part to the fact that SAR is a function of many different parameters, including device geometry, transmission frequency, amplifier power level, antenna location, etc. A typical stack-up configuration of a prior art cellular phone 70 is shown in FIG. 4. The phone 70 is encased in a housing 88 having a front face 84 and a back face 86. Components of the phone 70 include a circuit board 74 that holds processing circuitry (not shown) for controlling a display 72, a key pad 80, RF and other noisy, i.e., radiating, components 90 enclosed by canister-type shielding 76, and other device components (not shown). An antenna 82 is coupled through a feed point 92 to a transmitter and receiver on the circuit board 74. A battery 78 also is encased within the housing 88 for powering the components of the device 70.
Continuing with FIG. 4, the prior art stack-up configuration for the phone 70 is a display 72 on the front face 84 of the housing 88, which is stacked above the circuit board 74, which may have one or more canister-type shields 76 above and below the circuit board for shielding noisy components. The battery 78 is stacked below the circuit board, and typically is accessible through a battery door 94 on the back surface 86 of the housing 88. In addition to the canister-type shields 76 covering the noisy components residing on the circuit board, the phone 70 of the prior art requires additional shielding (not shown), referred to herein as compliance shielding, to bring the phone 70 into SAR compliance since high energy density appears immediately above the front face 84. This additional shielding may include foil, RF absorbing material, metalized plastic, and metal brackets, etc.
The shielding within the phone 70 that is used to comply with acceptable SAR levels continues to present a significant burden in the manufacture of the phone in terms of actual design efforts, iteration of compliance testing, and costs for the additional compliance shielding. The above described challenges are amplified as phone sizes decrease. Thus, it is desirable to continue innovations in the industry that lower the SAR level of the phone before the addition of the compliance shielding, and that reduce final costs of the phone in design and material costs.